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My Nest thermostat properly controls my HVAC system for heating and cooling, but is unable to turn on just the fan. I tried manually touching the green wire to red, and I can see a tiny spark and hear something faintly clunking up in the ceiling, but the fan motor never turns on.

What are my next steps in troubleshooting this?

Image of Nest thermostat wiring showing C, G, R, Y1, and W1 wired with blue, green, red, yellow, and white wires, respectively.

Nest thermostat configuration screen showing C, G, R, Y1 and W1 as common, fan, power, cool, and heat, respectively.

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    Did the fan work with the previous thermostat?
    – manassehkatz-Moving 2 Codidact
    Commented Oct 5, 2023 at 21:53
  • Probably going to need more details from the equipment end. Look for signs that the t-stat cable may be spliced somewhere between the 'stat and the furnace. Check how the conductors and terminals are matched up in the furnace. Maybe have somebody activate the fan mode at the 'stat while you listen for the clunk at the furnace.
    – Greg Hill
    Commented Oct 5, 2023 at 22:32
  • So the fan blows in heat or cool mode? Do you have access to the other end of the t-stat wire, and can you show us a pic of how things are connected there?
    – Huesmann
    Commented Oct 6, 2023 at 13:17
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    Does this answer your question? Why isn't my furnace fan running when the thermostat is calling for it? "I can see a tiny spark and hear something faintly clunking up in the ceiling" then no.... jumper R and G inside the furnace.
    – Mazura
    Commented Oct 7, 2023 at 18:05

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The next step would be to find the other end of the wires at the HVAC device and see how they are connected, and/or what's available to connect them to. If connecting green to red does not run the fan, then the wiring isn't correct, somewhere. People can and do borrow a wire they consider unimportant to run "some other function" on various thermostats of various levels of "not all that standardized, really."

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